1. Field of the Invention
This invention relates generally to circuits and methods for measuring a voltage, and more specifically to a circuit and method for measuring a voltage within an integrated circuit.
2. Related Art
Most known circuits and methods for measuring a voltage that exists within an integrated circuit either directly measure the voltage being measured or measure another voltage that is related in some way to the voltage being measured.
Known circuits and methods for testing a voltage that normally exists solely within a packaged integrated circuit, such as an output voltage of a charge pump, typically use one or more test pads that are disposed on a die of the integrated circuit. Switches, controlled by testing logic, are typically used to couple an output terminal of the charge pump to one of the test pads. As a result, the output voltage of the charge pump appears at such test pad. Then, the output voltage is directly measured using testing instrumentation that is located outside of the integrated circuit and coupled to such test pad.
A test pad usually has structures to protect circuitry within the integrated circuit from electrostatic discharge (ESD). An ESD protection structure of a low-voltage test pad usually includes a diode coupled between the test pad and a power supply voltage VDD of the integrated circuit. Because the output voltage of a charge pump is usually higher than VDD, known methods cannot use a low-voltage test pad to directly measure the output voltage of a charge pump. Instead, known methods disadvantageously require that a high-voltage test pad be used. A high-voltage test pad is a test pad capable of tolerating voltages higher than VDD. The ESD protection structure of a high-voltage test pad includes a zener diode coupled between the high-voltage test pad and a voltage source that provides a voltage at least as high as the output voltage of the charge pump. A high-voltage test pad occupies a larger area than a low-voltage test pad because a high-voltage test pad requires a greater amount of isolation from other circuitry within the integrated circuit. Also, zener diodes associated with a high-voltage test pad occupy a larger area than diodes associated with a low-voltage test pad. A high-voltage test pad disadvantageously has a greater amount of leakage current than a low-voltage test pad.
One known method of measuring the output voltage of a charge pump disposed within an integrated circuit is to use a switch to couple an output terminal of the charge pump to a test pad at which the output voltage is directly measured. This method requires a high-voltage test pad, rather than a low-voltage test pad, because the relatively high output voltage of the charge pump appears at the test pad. Because of the large amount of current leakage that is inherent in a high-voltage test pad, this method is typically limited to testing a charge pump that has high output current capability, e.g., about 100 μA. When this method is used for testing a charge pump that has low output current capability, e.g., about 10 μA, the measurement of the output voltage is likely to be inaccurate because the leakage current may constitute a large portion of the total output current capacity of the low current output charge pump.
Another known method of measuring the output voltage of a charge pump disposed within an integrated circuit is to use a converter, which is circuitry coupled between an output terminal of the charge pump and a test pad. The converter changes the output voltage of the charge pump to a lower voltage and the lower voltage is directly measured at the test pad. The converter comprises several components, and, therefore, occupies a large area. Also, the converter disadvantageously draws current from the charge pump, which may adversely affect measurement of the output voltage of the charge pump.
Another known method of measuring the output voltage of a charge pump disposed on an integrated circuit is to use an operational transconductance amplifier (OTA) that acts as a buffer between the output of a low output current capacity charge pump and a test pad at which a voltage outputted by the buffer is directly measured. The OTA has a higher output current capacity than the low output current capacity charge pump. The OTA typically outputs a voltage equal to the output voltage of the charge pump; therefore, this method requires a high-voltage test pad, rather than a low-voltage test pad. Disadvantageously, the OTA comprises more than one transistor; the OTA suffers from a voltage offset problem may adversely affect measurement of the output voltage of the charge pump; and there is a need to compensate for a feedback loop around the OTA.